Aluminum nitride powder and aluminum nitride sintered compact

ABSTRACT

To provide an aluminum nitride powder and an aluminum nitride sintered body which satisfy both high thermal conductivity of an aluminum nitride sintered body and reduction in the shrinkage factor at the time of sintering. 
     An aluminum nitride powder characterized in that it has local maximum values in size in regions of from 3 to 15 μm, from 0.5 to 1.5 μm and 0.3 μm or less, the proportions of particles in the respective regions are from 40 to 70%, from 25 to 40% and from 0.5 to 20% on the volume basis, and it has an oxygen amount of from 0.5 to 1.5 mass %. An aluminum nitride sintered body which is a sintered body of a powder mixture containing the above aluminum nitride powder and a sintering aid, characterized by having a thermal conductivity of at least 190 W/m·K and a shrinkage factor represented by the percentage of {(dimensions of the molded body before sintering)−(dimensions of the sintered body after sintering)}/(dimensions of the molded body before sintering) of at most 15%.

TECHNICAL FIELD

The present invention relates to an aluminum nitride powder and analuminum nitride sintered body.

BACKGROUND ART

Heretofore, along with high integration of a circuit board to beutilized for e.g. a powder module, heat generated by a semiconductordevice tends to increase. In order that the heat be effectivelydissipated, various methods have been studied, and ceramics such asalumina, beryllia, silicon nitride and aluminum nitride have beenutilized. Among them, aluminum nitride is a suitable material in view ofhigh thermal conductivity, high insulating properties, harmfulness,etc., and in addition, it has attracted attention since it has plasmaresistance and a coefficient of thermal expansion close to that ofsilicon in recent years, and has been used as a jig for a semiconductorproduction apparatus in the form of a single substance, as embedded in ametal heater, as fixed to a metal, etc. In any of these applicationforms, an aluminum nitride sintered body having a high degree ofparallelization and being less likely to be warped has been desired. Inorder to obtain such an aluminum nitride sintered body, it is importantto produce an aluminum nitride sintered body having small sinteringshrinkage. Here, the sintering shrinkage means a phenomenon such thatthe dimensions of the sintered body after sintering are smaller than thedimensions of the molded body before sintering, and the sinteringshrinkage becomes necessarily small when a high filing rate of a powdercan be achieved and the density of the molded body before sintering canbe increased.

Heretofore, for production of an aluminum nitride powder for productionof an aluminum nitride sintered body, an alumina reduction method and amethod of direct nitriding of a metal aluminum powder have been commonlyemployed, but these methods have both merits and demerits. An aluminumnitride powder obtainable by the alumina reduction method has a uniformparticle size and a small oxygen amount as compared with the directnitriding method, whereby it is easily sintered to produce a sinteredbody having a high thermal conductivity, but the shrinkage factor at thetime of sintering tends to be large, warpage or deformation is likely tooccur, and the production cost tends to be high. On the other hand, inthe direct nitriding method, the aluminum nitride powder will be easilyproduced at a low cost, but since the method comprises a grinding step,the obtained aluminum nitride powder tends to contain an increasedamount of impurities such as oxygen and the thermal conductivity canhardly be higher than that achieved by the alumina reduction method.Further, neither of the aluminum nitride powders obtained by theseproduction methods has been able to sufficiently achieve both higherthermal conductivity of the aluminum nitride sintered body and reductionin the shrinkage factor at the time of sintering.

The present applicant has proposed (Patent Document 1) that forproduction of an aluminum nitride sintered body which achieves both highthermal conductivity and small sintering shrinkage, an aluminum nitridepowder having a specific particle size and a specific oxygen amount maybe used, and that such an aluminum nitride powder can be prepared byproducing several types of aluminum nitride powders differing in theoxygen amount and the particle size and suitably combining them.

Patent Document 1: Japanese Patent No. 3403500

DISCLOSURE OF THE INVENTION OBJECT TO BE ACCOMPLISHED BY THE INVENTION

It is an object of the present invention to provide an aluminum nitridepowder which can achieve both high thermal conductivity of an aluminumnitride sintered body and reduction in the shrinkage factor at the timeof sintering, and an aluminum nitride sintered body, for example, analuminum nitride sintered body having a thermal conductivity of at least190 W/m·K and a shrinkage factor at the time sintering of at most 15%.

MEANS TO ACCOMPLISH THE INVENTION

The present inventors have conducted extensive studies and as a result,they have found an aluminum nitride powder and an aluminum nitridesintered body which achieve the above object and found a process forproducing an aluminum nitride powder to be used for production of thealuminum nitride sintered body.

Namely, the present invention provides the following.

(1) An aluminum nitride powder characterized in that it has localmaximum values in size in regions of from 3 to 15 μm, from 0.5 to 1.5 μmand 0.3 μm or less, the proportions of particles in the respectiveregions are from 40 to 70%, from 25 to 40% and from 0.5 to 20% on thevolume basis, and it has an oxygen amount of from 0.5 to 1.5 mass %.

(2) An aluminum nitride non-fired molded body characterized bycomprising a molded body of a powder mixture containing the aluminumnitride powder as defined in the above (1) and a sintering aid.

(3) An aluminum nitride sintered body which is a sintered body of thealuminum nitride non-fired molded body as defined in the above (2),characterized by having a thermal conductivity of at least 190 W/m·K anda shrinkage factor represented by the percentage of {(dimensions of themolded body before sintering)−(dimensions of the sintered body aftersintering)}/(dimensions of the molded body before sintering) of at most15%.

(4) The aluminum nitride sintered body according to the above (3), whichcontains the sintering aid in an amount of from 1 to 5 parts by mass per100 parts by mass of the aluminum nitride powder.

(5) The aluminum nitride sintered body according to the above (3) or(4), wherein the sintering aid is yttrium oxide or calcium oxide.

(6) A process for producing the aluminum nitride powder as defined inthe above (1), which comprises dispersively mixing a raw materialaluminum powder having an average particle size of at most 40 μm and anoxygen amount of at most 0.5 mass % with a nitrogen gas in a proportionof at most 100 g per 1 Nm³ of the nitrogen gas, atomizing the gas into areaction tube for nitriding, and collecting the product in a collectionsystem, characterized in that the oxygen concentration at a portion atwhich the temperature will be at least 100° C. in the reaction tube andthe collection system is controlled to be at most 100 ppm, and theproduct is taken out at a temperature of at most 100° C.

(7) The process according to the above (6), wherein the formed aluminumnitride powder has a BET specific surface area of at least 10 m²/g and avalue of the oxygen amount (mass %)/the specific surface area (m²/g) offrom 0.1 to 0.2.

EFFECTS OF THE INVENTION

According to the present invention, an aluminum nitride powder and analuminum nitride sintered body, which are excellent in both high thermalconductivity of the aluminum nitride sintered body and reduction in theshrinkage factor at the time of sintering, are provided. Particularly,an aluminum nitride sintered body having a thermal conductivity of atleast 190 W/m·K and a shrinkage factor at the time of sintering of atmost 15% is provided. Further, according to the present invention, anovel process for producing an aluminum nitride powder to be used forproduction of the aluminum nitride sintered body is provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram illustrating one example of an apparatus forproduction of an aluminum nitride powder.

MEANING OF SYMBOLS

1: feeder for aluminum powder

2: mixer

3: nozzle

4: reaction tube made of boron nitride

5: high frequency power source

6: graphite heating element

7: heat insulating material comprising porous carbon beads

8: quartz tube

9: bag filter

10: blower

11: temperature-sensing element made of glassy carbon

12: oximeter

13: oximeter

14: closed nitrogen circulating line

15: flow meter

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have conducted extensive studies on the particlesize construction and the oxygen amount of an aluminum nitride powderfor production of an aluminum nitride sintered body having a thermalconductivity of at least 190 W/m·K and a shrinkage factor at the time ofsintering of at most 15% and as a result, they have found the following.That is, in the above direct nitriding method for example, a metalaluminum powder is atomized in a furnace at high temperature in anitrogen atmosphere and nitrided, and the obtained aluminum nitridepowder is classified without being ground to produce aluminum nitridepowders having various particle size distributions and they are suitablecombined to achieve a specific particle size constitution, whereby thethermal conductivity will be high and the shrinkage factor at the timeof sintering will be low.

The aluminum nitride powder to be used in the present invention ispreferably an aluminum nitride powder produced by atomizing a metalaluminum powder in a furnace at high temperature in a nitrogenatmosphere for nitriding, particularly preferably one produced byinjecting a metal aluminum powder from the top portion of a reactiontube having a nitrogen atmosphere heated at 1,850° C. or higher fornitriding. The details are disclosed in JP-A-2003-119010 for example.Using the aluminum nitride powder produced by the above method, variousaluminum nitride powders differing in the particle size constitution andthe oxygen amount can be obtained by changing the settings of acentrifugal wind power classifier for example, and the powders aresuitable mixed considering the particle size constitution and the oxygenamount, whereby the aluminum nitride powder of the present invention canbe obtained.

A conventional aluminum nitride powder produced by the alumina reductionmethod or the direct nitriding method may be used as one component toprepare the aluminum nitride powder of the present invention, but thealuminum nitride powder of the present invention cannot be produced byeach aluminum nitride powder by itself, since it is difficult to produceparticles of 10 μm or larger by the alumina reduction method, and theoxygen amount will be large by the direct nitriding method.

In the present invention, the particle size distribution can be measuredby a laser diffraction method by measuring the frequency and thecumulative value of the volume distribution. The aluminum nitride powderof the present invention has local maximum values in size in regions offrom 3 to 15 μm (hereinafter, particles having particle sizes in thisregion will be referred to as “coarse particles”), from 0.5 to 1.5 μm(hereinafter, particles having particle sizes in this region will bereferred to as “medium particles”) and 0.3 μm or less (hereinafter,particles having particles sizes in this region will be referred to as“fine particles”). These local maximum values can be determined by thefrequency of the volume distribution, and the proportions of particlescan be determined by the cumulative values in the respective regions.

If the local maximum value for the coarse particles exceeds 15 μm, thesintering properties will be adversely affected, whereby the thermalconductivity will not improve. On the other hand., if it is smaller than3 μm, favorable sintering properties will be achieved, but the shrinkagefactor at the time of sintering will be large. If the proportion of thecoarse particles is less than 40%, the shrinkage factor at the time ofsintering will be large, and if it exceeds 70%, the sintering propertieswill be adversely affected, whereby the thermal conductivity will notimprove. Particularly preferably, the local maximum value for the coarseparticles is from 5 to 10 μm, and the proportion of these particles isfrom 50 to 65%.

If the local maximum value for the medium particles is larger than 1.5μm, since the particles sizes tend to be close to the local maximumvalue for the coarse particles, the sintering properties will beadversely affected, whereby the thermal conductivity will not improve.Further, if the local maximum value is smaller than 0.5 μm, since theparticle sizes tend to be close to the local maximum value for the fineparticles, the shrinkage factor at the time of sintering tends to belarge, and the oxygen amount tends to increase as well, wherebyachievement of high thermal conductivity will be adversely affected. Ifthe proportion of the medium particles is less than 25%, the sinteringproperties will be adversely affected, and if it exceeds 40%, theshrinkage factor at the time of sintering tends to be large.Particularly preferably, the local maximum value for the medium powderis from 1.3 to 0.8 μm, and the proportion of these particles is from 25to 35%.

If the local maximum value for the fine particles is larger than 0.3 μm,since the particle sizes tend to be close to the local maximum value forthe medium powder, the shrinkage factor at the time of sintering will belarge. If the proportion of the fine particles exceeds 20%, the oxygenamount tends to increase, and the thermal conductivity will be adverselyaffected. If it is less than 0.5%, the effect by the presence of thefine particles tends to be small, and the shrinkage factor at s the timeof sintering will be large. Particularly preferably, the local maximumvalue for the fine particles is from 0.25 to 0.05 μm, and the proportionof these particles is from 5 to 15%.

The total amount of the above coarse particles, medium particles andfine particles is preferably 100%, but is not necessarily 100%, andanother aluminum nitride powder may be contained so long as the aboveproportions of the particles are satisfied. The aluminum nitride powderof the present invention has an oxygen amount of from 0.5 to 1.5 mass %.If the oxygen amount is larger than 1.5 mass %, the sintering propertieswill be adversely affected and the thermal conductivity will notimprove, and if it is smaller than 0.5 mass % also, the sinteringproperties will be adversely affected. The oxygen amount is particularlypreferably from 0.8 to 1.3 mass %.

The aluminum nitride non-fired molded body of the present invention isone obtained by molding a powder mixture containing the aluminum nitridepowder of the present invention and a sintering aid. Further, thealuminum nitride sintered body of the present invention is one obtainedby sintering the above aluminum nitride non-fired molded body. Thesintering aid to be used in the present invention may, for example, bepreferably a compound of an alkaline earth metal or a compound of atransition metal. Specifically, it may, for example, be an oxide, afluoride, a chloride, a nitrate, a sulfate, a carbonate or the like ofan alkaline earth metal (such as Ca, Ba or Sr) or a transition metal(such as Y, La, Sc, Pr, Ce, Nd or Gd). Among them, preferred is yttriumoxide or calcium oxide. The sintering aid brings about high thermalconductivity in such a manner that it reacts with oxygen in the aluminumnitride powder i.e. an aluminum oxide to form a liquid phase of acomposite oxide (such as 2Y₂O₃.Al₂O₃, Y₂O₃.Al₂O₃ or 3Y₂O₃.5Al₂O₃), thisliquid phase increases the density of the sintered body and at the sametime, extracts e.g. oxygen as an impurity in the aluminum nitrideparticles and makes it be segregated as an oxide phase at the crystalgrain boundary. If the amount of the sintering aid to be used is small,the liquid phase sintering tends to be insufficient, and if it is large,the proportion of the crystal grain boundary tends to be high, and thethermal conductivity will not increase in either case. In the presentinvention, the amount of the sintering aid to be used is preferably from1 to 5 parts by mass per 100 parts by mass of the aluminum nitridepowder.

To mix the aluminum nitride powder with the sintering aid, a ball millor a rod mill may, for example, be used. The powder mixture may bemolded as it is, or may be molded after it is granulated by e.g. a spraydryer method or a tumbling granulation method. Molding may be carriedout, for example, by a dry press molding method or a cold isostaticalpress molding method (CIP method) alone or in combination. The presspressure in dry press molding is preferably from 50 to 300 MPa,particularly preferably from 100 to 250 MPa. In either of the dry pressmolding method and the CIP method, an organic binder is used as the caserequires. Further, it is possible to mix the aluminum nitride powder,the sintering aid, the organic binder, and as the case requires, aplasticizer, a disperser or the like, and subject the resulting mixtureto extrusion, doctor blade molding or the like.

As the organic binder, polyvinyl butyral, polyacrylate, polymethylmethacrylate, methylcellulose, polyethylene, wax, etc. may be used. In acase where an organic binder is used, the molded body before sinteringis heated in a stream of a nitrogen gas, the air or the like at from 350to 700° C. for from 1 to 10 hours to remove the organic binder(debindering).

The molded body is then fired. Firing is preferably carried out, forexample, by holding the molded body in a non-oxidizing atmosphere suchas in a nitrogen gas or an argon gas in a temperature range of from1,600 to 1,900° C. for from 1 to 10 hours, particularly preferably from2 to 7 hours. If the firing temperature is less than 1,600° C., thesintering tends to be insufficient, whereby production of an aluminumnitride sintered body having a thermal conductivity of at least 190 W/mKtends to be difficult. Further, if the firing temperature exceeds 1,900°C., the vapor pressure of aluminum nitride in the furnace tends to behigh, whereby densification will hardly be achieved. The holding time ispreferably the shortest time over which the density of the sintered bodycan be made to be 98% or higher within the above temperature range. Thisis because if the molded body is fired for a long time in a temperaturerange in which the density of the sintered body will be 98% or higher,the AlN particles will grow more than necessary to form coarse grains,whereby the volume of the two grain boundaries tends to be relativelysmall as compared with the triple point, and the grain boundary phasetends to be segregated at the triple point in a large amount as comparedwith the two grain boundaries, and further, the liquid phase of thealuminum composite oxide tends to exude on the surface of the sinteredbody.

The aluminum nitride powder to be used for production of the abovealuminum nitride sintered body is produced preferably by the followingproduction process. This process comprises diluting an aluminum powderhaving a low oxygen content to a low concentration with a nitrogen gas,atomizing the mixture into a reaction tube having the oxygenconcentration controlled for nitriding, and collecting the product in acollection system similarly having the oxygen concentration controlled.Now, one example of the production process will be described in furtherdetail with reference to the drawing.

FIG. 1 is a diagram illustrating one example of an apparatus forproduction of an aluminum nitride powder. A raw material aluminum powderis supplied in a certain amount to a mixer 2 by means of a feeder 1 foraluminum powder, such as a table feeder or a screw feeder. In the mixer,the aluminum powder is mixed with a nitrogen gas, and the mixture isatomized into a reaction tube 4 made of boron nitride through a nozzle3. As the nozzle, a ring nozzle may, for example, be used. Around theperiphery of the reaction tube, a graphite heating element 6 is disposedto keep a predetermined temperature, and the reaction tube is heated bya high frequency power source 5. The graphite heating element isheat-insulated with a heat insulating material 7 comprising porouscarbon beads and supported by a quartz tube 8. The reaction temperatureis measured by means of an optical pyrometer employing atemperature-sensing element 11 made of glassy carbon provided at acenter portion of the heating element.

The product (aluminum nitride powder) is drawn together with thecirculating nitrogen gas in a closed nitrogen circulating line 14 fromthe furnace bottom portion by a blower 10 and collected in a bag filter9. The oxygen amount in the reaction tube and the collection system ismonitored by oximeters 12 and 13, respectively provided at thedownstream portion of the reaction tube and at the closed nitrogencirculating line. Infiltration of the air from the outside is prevented,and the oxygen amount in the reaction tube and the collection tube iscontrolled, by controlling the nitrogen purity, the air tightness of thereaction tube and the bag filter and the internal pressure of thereaction tube by the balance between the amount of the circulatingnitrogen gas and the suction power of the blower, specifically, bykeeping the internal pressure to be in a slightly pressurized state (ata level of from 5 to 10 mmAq). Here, the closed nitrogen circulatingline means the entire collection system including the bag filter and theblower.

If the average particle size of the raw material aluminum powder to beused in the present invention is large, aluminum will not sufficientlyevaporate and unreacted aluminum may remain, and thus the averageparticle size is at most 40 μm, particularly at most 30 μm. Further, thesurface oxide film will be included in the interior of the productaluminum nitride powder, and thus the oxygen amount in the raw materialaluminum powder is at most 0.5 mass %, preferably at most 0.4 mass %.Preferred is an atomized powder with which the risk of explosion can bereduced. Further, if the concentration of the aluminum powder in thenitrogen gas is high, spatial dispersion of aluminum particles tends tobe poor, and the probability of cohesion between particles tends to behigh, whereby formation of an aluminum nitride powder in the form offine particles may be inhibited, and thus the concentration is at most100 g, preferably from 50 to 80 g in 1 Nm³ of the nitrogen gas.

The temperature of the reaction tube is preferably from 1,900 to 2,200°C. If it is lower than 1,900° C., the aluminum powder will hardlyevaporate, and if it is higher than 2,200° C., formation of fibers ofaluminum nitride will take place by priority.

The formed aluminum nitride powder is carried in the closed nitrogencirculating line 14 by an unreacted nitrogen gas and the nitrogen gascirculating in a closed manner by the blower, and collected by acollecting apparatus such as the bas filter 9. What is important is tobring the oxygen concentration at all portions at which the temperaturewill be at least 100° C. to be at most 100 ppm and to take the productout at a temperature of at most 100° C. If any of these requirements isnot met, preparation of an aluminum nitride powder to be used forpreparation of an aluminum nitride powder having the above specificparticle size and specific oxygen amount will be difficult.Particularly, preparation of an aluminum nitride powder having a BETspecific surface area of at least 10 m²/g and a value of the oxygenamount (mass %)/the specific surface area (m²/g) of from 0.1 to 0.2 willnot be carried out. Here, the aluminum nitride powder having a BETspecific surface area of at least 10 m²/g and a value of the oxygenamount (mass %)/the specific surface area (m²/g) of from 0.1 to 0.2 is apowder in the form of fine particles (i.e. having an increased specificsurface area) required for molding at a high filling rate, and is apowder having an oxygen amount which increases as the powder becomes inthe form of fine particles and adversely affects the thermalconductivity, suppressed.

In order that no excessive oxide layer will form on the surface of theparticles except for the “natural oxide film” which necessarily formswhen the aluminum nitride powder is exposed to the air at roomtemperature, in the present invention, it is required that the oxygenconcentration is at most 100 ppm, preferably at most 10 ppm, at allportions at which the temperature will be at least 100° C. at which theoxidation reaction may take place. In this viewpoint, it is veryimportant to take out the product while the internal temperature of thecollection apparatus such as the bag filter is kept at 100° C. or lower.It is preferred to employ e.g. a double damper structure for the removalmechanism so that the air will not infiltrate into the collectionapparatus such as the bag filter when the product is taken out.

One example of a method for preparing an aluminum nitride powder capableof producing an aluminum nitride sintered body which satisfies both highthermal conductivity and low sintering shrinkage by using the aluminumnitride powder produced by the present invention, is shown below.Namely, the aluminum nitride powder A (having a BET specific surfacearea of 20 m²/g, an oxygen amount of 2.2 mass % and a value of theoxygen amount (mass %)/the specific surface area (m²/g) of 0.11)produced by the present invention, another aluminum nitride powder B(having a BET specific surface area of 5 m²/g, an oxygen amount of 0.8mass %) and another aluminum nitride powder C (having a BET specificsurface area of 1 m²/g and an oxygen amount of 0.6 mass %) are mixed ina mass ratio of 10:30:60. The aluminum nitride powder thus prepared hasa mold density of at least 70% and when sintered at from 1,750 to 1,850°C., the shrinkage factor represented by the percentage of {(dimensionsof the molded body before sintering)−(dimensions of the sintered bodyafter sintering)}/(dimensions of the molded body before sintering) willbe so low as 12% (usually 16%). Further, the oxygen amount can besuppressed to be 1.0 mass %, whereby a thermal conductivity of 190 W/m·Kwill be very easily realized.

EXAMPLES Examples 1 to 16 and Comparative Examples 1 to 13

From the top of a reaction tube in a nitrogen gas atmosphere kept at1,950° C., a raw material aluminum powder (purity: 99.97 mass %, averageparticle size: 25 μm) was atomized under a condition of 2 kg/hremploying a nitrogen gas as a carrier gas. On the other hand, a nitrogengas as a reaction gas was supplied in a total amount including the abovenitrogen gas as the carrier gas of 200 l/min to prepare an aluminumnitride powder, which was drawn at the furnace lower portion by a blowerand collected by a bag filter.

The aluminum nitride powder was classified by a centrifugal wind powerclassifier to obtain various aluminum nitride powders differing in theparticle size constitution and the oxygen amount. Namely, various coarseparticles (classification yield: 10 to 20%) having oxygen amounts offrom 0.4 to 0.8 mass % and grain sizes of from 3 to 15 μm, variousmedium particles (classification yield: 50 to 70%) having oxygen amountsof from 0.9 to 1.8 mass % and grain sizes of from 0.5 to 1.5 μm andvarious fine particles having oxygen amounts of from 1.8 to 2.6 mass %and grain sizes of 0.3 μm or less, were produced. These powders weresuitably combined to prepare various aluminum nitride powders differingin the oxygen amount, having a local maximum value P1 in a region offrom 3 to 15 μm, a local maximum value P2 in a region of from 0.5 to 1.5μm and a local maximum value P3 in a region of 0.3 μm or less, as shownin Tables 1 and 2.

To 100 parts by mass of each of the obtained aluminum nitride powders, asintering aid (first class reagent, average particle size: about 0.7 μm)in parts by mass as identified in Tables 1 and 2 and 3 parts by mass ofan organic binder (polyacrylate) were added and mixed in a wet ball millemploying methanol as a dispersion medium for 3 hours, followed byfiltration and drying. Then, press molding under a pressure of 200 MPawas carried out to obtain an aluminum nitride non-fired molded body of50 mm×50 mm×5 mm, and the relative density (1) of the molded body wasmeasured. Then, the molded body was put in a crucible made of boronnitride (BN) and heated in a nitrogen gas at 600° C. for 2 hours fordebindering, and then put in a firing furnace and subjected to normalpressure sintering in a nitrogen gas atmosphere at 1,780° C. for 6 hoursto prepare an aluminum nitride sintered body. With respect to theproduct, the relative density (2) and the thermal conductivity (3) ofthe sintered body were measured and further, the shrinkage factor (4) atthe time of sintering was measured as follows. The results are shown inTables 1 and 2.

(1) Relative density of the aluminum nitride non-fired molded body:obtained by dividing the total mass of the aluminum nitride powder andthe sintering aid by the volume of the aluminum nitride molded body, anddividing the obtained value by the theoretical density of the aluminumnitride sintered body having the content of the sintering aid added. Themass of the aluminum nitride powder and the sintering aid was obtainedfrom the amount of use at the time of raw material preparation.

(2) Relative density of the aluminum nitride sintered body: obtained bydividing the density of the sintered body determined by the Archimedesmethod by the theoretical density of the aluminum nitride sintered bodyhaving the content of the sintering aid added.

(3) Thermal conductivity of the aluminum nitride sintered body: measuredby means of a laser flash thermal constants analyzer (“TC-7000”manufactured by Shinku Riko K. K.) by preparing a circular test specimen(diameter: 25 mm×1.5 mm).

(4) Shrinkage factor at the time of sintering aluminum nitride: thelongest direction (e.g. the diagonal direction in the case of arectangular or the major axis direction in the case of an ellipse) ofthe molded body and the sintered body was measured, the average of thelengths in optional four directions was determined to calculate theshrinkage factor from the formula: shrinkage factor (%)={(dimensions ofthe molded body before sintering)−(dimensions of the sintered body aftersintering)}×100/(dimensions of the molded body before sintering).

The particle size distribution was measured by using a laser diffractionscattering type measuring apparatus (“LS-230” manufactured by BeckmanCoulter K.K.), and the oxygen amount was measured by using anoxygen/nitrogen simultaneous analyzer manufactured by HORIBA, Ltd.

TABLE 1 Aluminum nitride powder Local Local Local Local Local Localmaximum maximum maximum maximum maximum maximum Oxygen value P1 value P2value P3 value P1 value P2 value P3 amount μm μm μm % % % (mass %) Ex. 115 1 0.1 60 30 10 0.87 Ex. 2 10 1 0.1 60 30 10 0.96 Ex. 3 3 1 0.1 60 3010 1.11 Ex. 4 10 1 0.1 70 25 5 0.80 Ex. 5 10 1 0.1 40 40 20 1.26 Ex. 610 0.5 0.1 60 30 10 1.09 Ex. 7 10 1 0.1 60 30 10 0.96 Ex. 8 10 1.5 0.160 30 10 0.82 Ex. 9 10 1 0.1 59.5 40 0.5 0.81 Ex. 10 10 1 0.1 55 25 201.15 Ex. 11 10 1 0.3 60 30 10 0.86 Ex. 12 10 1 0.15 60 30 10 0.94 Ex. 133 0.5 0.15 40 40 20 1.50 Ex. 14 15 1.5 0.3 70 29 1.0 0.50 Ex. 15 3 0.50.15 40 40 20 1.50 Ex. 16 15 1.5 0.3 70 29 1.0 0.50 Aluminum nitridenon-fired Sintering aid molded body Aluminum nitride sintered bodyAmount Relative Relative Shrinkage Thermal (parts by density densityfactor conductivity Type mass) % % % W/m · K Ex. 1 Y203 3 70 100 12 205Ex. 2 Y2O3 3 68 100 13 202 Ex. 3 Y2O3 3 66 100 14 200 Ex. 4 Y2O3 3 69100 12 205 Ex. 5 Y2O3 3 66 100 15 200 Ex. 6 Y2O3 3 69 100 12 201 Ex. 7Y2O3 3 71 100 12 205 Ex. 8 Y2O3 3 70 100 13 207 Ex. 9 Y2O3 3 69 100 14210 Ex. 10 Y2O3 3 68 100 15 205 Ex. 11 Y2O3 3 69 100 13 206 Ex. 12 Y2O33 71 100 12 200 Ex. 13 Y2O3 5 65 100 15 195 Ex. 14 Y2O3 1 70 100 12 211Ex. 15 CaO 5 65 100 15 195 Ex. 16 CaO 1 70 100 12 209

TABLE 2 Aluminum nitride powder Local Local Local Local Local Localmaximum maximum maximum maximum maximum maximum Oxygen value P1 value P2value P3 value P1 value P2 value P3 amount μm μm μm % % % (mass %) Comp.Ex. 1 20 1 0.1 60 30 10 0.88 Comp. Ex. 2 2 1 0.1 60 30 10 1.12 Comp. Ex.3 10 1 0.1 80 15 5 0.73 Comp. Ex. 4 10 1 0.1 30 60 10 1.18 Comp. Ex. 510 2 0.1 60 30 10 0.81 Comp. Ex. 6 10 0.3 0.1 60 30 10 1.12 Comp. Ex. 710 1 0.1 70 20 10 0.88 Comp. Ex. 8 10 1 0.1 40 50 10 1.11 Comp. Ex. 9 101 0.4 60 30 10 0.85 Comp. 10 1 0.1 70 29.8 0.2 0.73 Ex. 10 Comp. 10 10.1 45 25 30 1.38 Ex. 11 Comp 3 0.5 0.15 35 40 25 1.59 Ex. 12 Comp. 151.5 0.3 85 14.8 0.2 0.42 Ex. 13 Aluminum nitride non-fired Sintering aidmolded body Aluminum nitride sintered body Amount Relative RelativeShrinkage Thermal (parts by density density factor conductivity Typemass) % % % W/m · K Comp. Ex. 1 Y2O3 3 73 90 — — Comp. Ex. 2 Y2O3 3 5899 16 187 Comp. Ex. 3 Y2O3 3 67 95 — — Comp. Ex. 4 Y2O3 3 65 99 16 187Comp. Ex. 5 Y2O3 3 70 97 — — Comp. Ex. 6 Y2O3 3 69 99 16 185 Comp. Ex. 7Y2O3 3 69 97 — — Comp. Ex. 8 Y2O3 3 68 99 16 188 Comp. Ex. 9 Y2O3 3 6999 16 189 Comp. Y2O3 3 70 99 16 187 Ex. 10 Comp. Y2O3 3 68 99 17 189 Ex.11 Comp. Y2O3 5 63 99 16 180 Ex. 12 Comp. Y2O3 1 68 89 — — Ex. 13

Examples 17 and 18 and Comparative Examples 14 and 15

Using the apparatus shown in FIG. 1, an aluminum nitride powder wasproduced under conditions shown in Table 1. The capacity of the reactionfurnace is 170 kVA and the output is 100 kW. The reaction tube 4 made ofboron nitride has an inner diameter of 200 mm and a total length of3,000 mm, and the quartz tube 8 has an inner diameter of 450 mm and atotal length of 3,000 mm. As the feeder 1 for aluminum powder, a screwfeeder was used. Infiltration of the air from the outside was preventedand the oxygen amount in the system was controlled to be at most 10 ppmby controlling the nitrogen purity, the air tightness of the reactiontube and the bag filter, and the internal pressure of the reaction tubeby the balance between the amount of the circulating nitrogen gas andthe suction power of the blower. Further, the product was taken outwhile the internal temperature of the bag filter was kept at 100° C. orlower.

With respect to the obtained aluminum nitride powder, the BET specificsurface area and the oxygen amount were measured by means of “QS16apparatus” manufactured by YUASA IONICS COMPANY, LIMITED and anoxygen/nitrogen simultaneous analyzer “model EMGA620W” manufactured byHORIBA, Ltd., respectively, to calculate the proportion of the oxygenamount (mass %)/the specific surface area (m²/g). The results are shownin Table 3.

TABLE 3 Aluminum powder Oxygen Concen- amount tration BET (%)/ Averagein Reaction Oxygen specific Oxygen specific particle Oxygen nitrogentube concentration surface amount surface size amount gas temperature inprocess area in AlN area (μm) (%) (g/Nm³) (° C.) (ppm) (m²/g) powder(m²/g) Ex. 17 30 0.4 100 2,000 10 10.2 1.3 0.127 Ex. 18 30 0.4 100 2,0005 11.2 1.2 0.107 Comp. 45 0.2 100 2,000 5 Unreacted Al remained Ex. 14Comp. 30 0.6 100 2,000 10 10.5 2.2 0.210 Ex. 15

INDUSTRIAL APPLICABILITY

The aluminum nitride powder of the present invention is useful as a rawmaterial for production of an aluminum nitride sintered body, a fillerfor a resin or a rubber, etc. Further, the aluminum nitride non-firedmolded body of the present invention is useful for production of analuminum nitride sintered body. Further, the aluminum nitride sinteredbody of the present invention is useful as a structural member, aradiator board, a ceramic substrate for a circuit board, etc.Particularly, it is suitable as a ceramic substrate for a module fore.g. an electric vehicle.

The aluminum nitride powder produced by the present invention may beused, for example, as one raw material for preparation of an aluminumnitride sintered body which satisfies both high thermal conductivity ofthe aluminum nitride sintered body and low sintering shrinkage.

The entire disclosure of Japanese Patent Application No. 2004-094567filed on Mar. 29, 2004 including specification, claims, drawing andsummary is incorporated herein by reference in its entirety.

1. An aluminum nitride powder comprising: from 40 to 70% of coarse particles having a size of 3 to 15 μm, from 25 to 40% of medium particles having a size of 0.5 to 1.5 μm, and from 0.5 to 20% of fine particles having a size of 0.3 μm or less, the percentages being on a volume basis, and wherein the aluminum nitride powder has an oxygen amount of from 0.5 to 1.5 mass %.
 2. An aluminum nitride non-fired molded body comprising a molded body of a powder mixture comprising the aluminum nitride powder as defined in claim 1 and a sintering aid.
 3. The aluminum nitride powder according to claim 1, wherein the coarse particles have a size of 5 to 10 μm and are present in an amount of from 50 to 65 volume %.
 4. The aluminum nitride powder according to claim 1, wherein the medium particles have a size of 0.8 to 1.3 μm and are present in an amount of from 25 to 35 volume %.
 5. The aluminum nitride powder according to claim 1, wherein the fine particles have a size of 0.05 to 0.25 μm and are present in an amount of from 5 to 15 volume %.
 6. The aluminum nitride powder according to claim 1, wherein the coarse particles have a size of 5 to 10 μm and are present in an amount of from 50 to 65 volume %, the medium particles have a size of 0.8 to 1.3 μm and are present in an amount of from 25 to 35 volume %, and the fine particles have a size of 0.05 to 0.25 μm and are present in an amount of from 5 to 15 volume %.
 7. The aluminum nitride powder according to claim 1, wherein the aluminum nitride powder has an oxygen amount of from 0.8 to 1.3 mass %.
 8. A process for producing an aluminum nitride powder comprising: dispersively mixing a raw material aluminum powder having an average particle size of at most 40 μm and an oxygen amount of at most 0.5 mass % with a nitrogen gas in a proportion of at most 100 g per 1 Nm³ of the nitrogen gas, atomizing the gas into a reaction tube for nitriding, and collecting, in a collection system, the aluminum nitride powder, said aluminum nitride powder comprising from 40 to 70% of coarse particles having a size of 3 to 15 μm, from 25 to 40% of medium particles having a size of 0.5 to 1.5 μm, and from 0.5 to 20% of fine particles having a size of 0.3 μm or less, the percentages being on a volume basis, and said aluminum nitride powder having an oxygen amount of from 0.5 to 1.5 mass %, wherein the oxygen concentration at a portion at which the temperature will be at least 100° C. in the reaction tube and the collection system is controlled to be at most 100 ppm, and the product is taken out at a temperature of at most 100° C.
 9. The process according to claim 8, wherein the formed aluminum nitride powder has a BET specific surface area of at least 10 m²/g and a value of the oxygen amount (mass %)/the specific surface area (m²/g) of from 0.1 to 0.2. 